Insulation of electrical devices



Oct. 14, 1958 H. L. SAUMS INSULATION OF ELECTRICAL DEVICES Filed Oct. 24, 1955 INVENTOR HARRY L. SAUMS ATTORNEYS FIG. 3

United States Patent INSULATION OF ELECTRICAL DEVICES Harry L. Saums, North Muskegon, Mich., assignor to Anaconda Wire and Cable Company, a corporation of Delaware Application October 24, 1955, Serial No. 542,259

7 Claims. (Cl. 310-45) This invention relates to the insulation of electrical devices. More particularly, it relates to the insulation of windings such as are used ,in transformers, dynamoelectric machines, and other electro-magnetic devices.

A Winding of this type consists essentially of a conductor wound to provide the desired number of turns, and the insulation associated therewith. The insulation generally comprises a coating of enamel applied over the conductor, an impregnant or varnish which fills the interstices of the winding, and overall insulating material enclosing the coil as a whole, or supporting it, or separating it from the structure by which it is supported. Heretofore, the material used for each of these insulation components has been selected on the basis of considerations applicable to that one component only, and without much consideration of the other insulating components.

For the coating of enamel applied over the conductor, an enamel is chosen which will adhere well to the conductor and will be resistant to damage by abrasion and will have sufficient flexibility so that it can withstand the coil forming operations without cracking. Enamels formed of natural resins and drying oils have been Widely used for this service. However, synthetic enamels having properties superior to those of the natural resin enamels have been developed and have replaced the natural resin enamels to a considerable extent. Among the synthetic enamels which have received acceptance are nylon, and combinations of polyvinyl resins and phenol aldehyde resins. Recently, enamels consisting of the reaction product of an epoxy resin and curing agents therefor have been used successfully in this service.

The impregnant is used to displace air present in the interstices of the winding, and may be an oil or a normally solid insulating material. By displacing the air the moisture resistance of the winding is increased and the entrance of foreign matter into the interstices of the winding is effectively prevented and heat dissipation is improved. When solid insulating material is used as the impregnant the ease with which the air may be displaced by the solid insulating material is a primary consideration and governs to a great extent the material chosen for the impregnant. Complete displacement of the air by solid insulating material is desirable as otherwise entrapped air or gas bubbles will thermally insulate the winding and will preclude realization of the full dielectric strength of the impregnant. Spirit varnishes comprising a solution of an alkyd resin or phenolic resin and a drying oil fatty acid in a suitable solvent are Widely used for this impregnating service. While such varnishes are generally satisfactory, they have the disadvantage that it is difiicult to completely expel the solvent from the winding and thereby leave the impregnant free from gas bubbles. Solventless varnishes do not have this disadvantage and are used for many applications. An example of a solventless varnish which has been used successfully is a solution of polyester resin and styrene. Upon heating a winding impregnated with such a solution the sty- Patented Oct. 14, 1958 rene reacts with the polyester resin to form a thermosetting plastic.

The overall insulating material which supports the coil or winding, or separates it from its surroundings, serves not only to insulate the coil or winding as a whole from adjacent structures but also to protect it from injury such as is likely to result from mechanical causes. Therefore, a dielectric which will provide a hard outer surface is usually selected. Thus, shellac or a phenolic resin applied by brushing, dipping or spraying may be used. Cast phenolic resins or cast urea formaldehyde resins are often used when a thick jacket is desired. When a hard outer surface is not required, plastic sheeting or a tape wrapping may be used. Thus, windings are often wrapped in polyethylene sheeting which is secured in place with adhesive tape. Fiber board sheeting is often used to form insulating coil support forms, or to separate coils from a metallic supporting structure, such as form the walls of a motor rotor or stator slot.

As a consequence of the considerations which normally control selecting insulation for the coating on the conductor, for the impregnant, and for the overall supporting or protective insulating material, each finished winding generally includes several different kinds of insulation. I have discovered that improved results are realized if the same insulating material, i. e. material of substantially the same chemical composition, is used for each of these insulation components. Accordingly, the present invention provides an electrical device having a winding characterized in that each of the insulating components associated therewith is of substantially the same chemical composition. The winding of the invention consists essentially of a conductor wound into a coil, and insulating components insulating the turns of the winding from each other and from the surroundings. The insulating components comprise a coating on the conductor, an impregnant filling the interstices of the winding, and an overall layer of protective insulating material. All three of these components, in accordance with this invention, are of essentially the same chemical composition and have substantially the same dielectric constant. Thus, the winding of the invention is free from interfaces between insulating materials of different composition, and electrical stress gradients within the insulating material associated with the winding are minified.

In general, any insulating material which is satisfactory for coating the conductor which is to form the turns of the winding and which can be applied to the winding in a liquid phase without adversely affecting insulation already present in the Winding as a solid phase and which will cure in fairly thick sections may be used in practicing the present invention. Synthetic insulating materials comprising the reaction product of an epoxy resin and at least one curing agent therefor are, however, preferred.

The invention may be better understood by reference to the accompanying drawings, in which:

Figure l is a perspective view, partly broken away, of a coil according to the invention;

Figure 2 is a view of a portion of a stator of a dynamoelectric machine having windings according to the invention; and

Figure 3 is a cross-sectional view of a slot of a stator such as is shown in Figure 2.

Referring to Figure 1, there is there shown a coil 5 formed of a conductor 6 wound to provide the desired number of turns on a core 7. The conductor is individually insulated with a film or coating 8, and the interstices between the turns of the coil are filled with an impregnant 9. A protective insulation covering 11 encloses the coil. The core 7, coating 8, impregnant 9, and jacket 11 are each formed of insulating material, and the coil is char acterized in that all of these insulating materials are of substantially the same chemical composition.

Windings of a dynamo-electric machine constructed according tothe inventionare shown in Figuresland. 3. There is there shown a portion of a dynamo-electric ma.- chine having a stationary element or. stator lscomprising a frame 16 anda magnetic core 17. formed'with poles 18 separatedby slots 19. Motor.windings.20,,21, 22.and 23 are. positioned. with portions thereof in. the. slots. 1-9. Wedges 24 inserted. after the windings have been placed inthe slots hold the windings in place. The ends of windings 20 and 21 are connected together and the resulting'junction is covered by sleeve 25 which is formed of insulating material and the ends. of windings22 and 23. are connected together and the resulting junction is covered by insulatingsleeve 26.

A typical Cl'OSS.-SCtlOI1 of one of the slots 19 is shown in: Figure 3. Referring to Figure 3 the. motor windings 21. and 23v are positioned in the slot 19 formed in the magnetic core 17. The motor winding 23 isformed of a wire 27 coated with a film or layer 28 of insulating material, and the interstices of the winding are filled with an impregnant 29. A slot liner 31 formed of insulating material separates the portion of the winding which is positioned in the slot-1S from the side of the polesdefining the slotv The motor running winding 21 is constructed in the same way and comprises a wire 36 covered by a film or layer of insulation 38, an-impregnant39, and is separated from the adjacent poles by a slot liner 41 (which may be the same slot liner 31 that protectsthewinding 23). The wedge 24 holds the windings in place inthe slot 19. In accordance with the invention, all of the insulating materials associated with these windings, including the films 28 and 38, the impregnants129 and 39, and the slot liners 3r and 41, are of substantially the same chemical composition.

The best materials which I have found for practicing the present invention are the products of reaction betweenan epoxy resin and one or more curing agents therefor. The most common of the epoxy resins arecondensation polymers of p,p'-isop1'opylidene-diphenol and epichlorohydrin. Epoxy polymers are characterized in that they have active groups (hydroxyl and epoxy groups) more or less widely spaced along the chain of carbon atoms forming the polymer molecules. Curing agents react with these active groups, cross-linking the epoxy resin polymers to form a thermosetting plastic; Curing agents which are well suited for use in practicing the invention are urea formaldehyde, melamine formaldehyde, and phenol formaldehyde resins. The epoxy resinused in formulating the resin mixture is preferably a high molecular weight epoxy resin having an epoxide equivalent (grams of resin containing one equivalent of epoxide') of 24-0l)3009. Such an epoxy resin is presently marketed under the name Epon 1009 by the Shell Chemical Corporation.

This insulating material may be applied to the conductor by a dip and bake process such as is well known to the art. Thus, the conductor is drawn through a liquid solution formed of a solvent having the epoxy resin and curing agent dissolved therein, then over rollers or wipers to remove the excess solution therefrom, and is then passed through an oven maintained at an elevated temperature wherein the solvent is evaporated and the various resins present are caused to react to form athermosetting plastic film on the conductor. The dip and bake process may be repeated if it is desired to build up a thicker coating than is conveniently applied in one clip and bake operation.

impregnation of the interstices betweenthe turns of the'coil with the same material may also be accomplished by known methods. Thus, impregnation may be carried out by the vacuum-pressure process wherein the interstices are first evacuated and the winding is then immersed in a bath containing the epoxy resin and curing agents therefor. In order that the impregnant be of the same composition as the coating on the conductor, the reactant resins present in the bath used for impregnating the winding preferably are the same and are present in about the same proportions as in the bath used for coating the conductor. While the coil is immersed pressure is applied over the bath in order to insure that the interstices are completely filled by the solution ofthe reactant resins. The winding is then removed from the bath and subjected to a baking operation to evaporate the solvent and cause i the resins to react.

The overall layer of insulation protecting the winding fromv its. surroundings maybe applied by potting or dipping and baking, or by spraying, or by brushing onto the impregnated winding, a solution having the same resin composition as that used to coat the conductor. If the overall layer is applied by dipping and baking, spraying, or brushing, it may be repeated as desired to obtain the desired layer thickness. It may also be applied in the form of a prefabricated filmor sheet or fabric composed of one or more layers or of filaments of the epoxy resin composition.

A core for use in forming a coil according to the invention may be obtained by coatingkraft paper, or a plastic tube or a fibrous fabric, with the same resin material as is used for the other insulation components of the coil, and sheet insulating material for use as slot liners, wrapping tapes, protective enclosures, etc., may be similarly prepared. This procedure has the disadvantage that, the paper fabric, or other base material introduces an insulating component of. different chemical construction than the coating resin into the overall layer of insulation. However, if such base material is thoroughly impregnated and does not constitute the bulk of the overall layer of insulation, it doesnot result in seriously impairing the dielectric match of the overall insulation with the conductor insulation and the impregnant.

Another insulating material which may be used to practice the present invention is the resinous copolymer product obtained by heating a mixture of polyvinyl acetal resin and a phenol aldehyde resin. This material is widely, used for wire enamel and is sold commercially under the trade name Formvar. A bath of these resins as. has been used heretofore for enameling wires may be used for constructing a winding according to the present invention. The conductor may be coated by dipping and baking, and impregnation of the winding may be carried out as described hereinbefore for impregnating a winding with an epoxy insulating material. Sheet or tape components of the same resinous material may be used for the overall protection insulation.

Nylonv is also used as a wire enamel and may be used to construct a .coil according to the invention. In enameling wire, nylon is commonly applied in either oftwo ways. The wire to be coated may be dipped in a bath comprising a solution of nylon in a solvent, e. g. formic acid, and then passed through an oven where the solvent evaporates so that the nylon is left in film form on the wire; or thewire to becoated is. dipped in a bath of molten nylon and is then dipped in water so that the molten nylon picked up in the nylon bath solidifies about the wire. Nylon coatings may also be formed about the wire by extruding operations. In practicing the present invention either a' solution of nylon'or molten nylon may be usedto coat the conductor, and also to impregnate the winding. Any nylon composition suitable for enameling conductors, e. g. polyhexamethylene adipamide, may be used in constructing a coil of the invention. Nylon is readily available in solid sheet and fabric forms, and in such forms it may be used to form the overall insulation. Also a core for use in constructing a coil according to the invention may be cast from nylon ofthe composition being used.

An oleo-resinous insulating material of the type which has beenused heretofore to enamel conductors-may also be used to construct windings according to the invention. In using these materials a drying oil and a natural resin are cooked together and the cooked material is then dissolved in a hydrocarbon solvent such as naphtha. Coating of the conductor, impregnation of the coil, and formation of overall protective insulation about the winding may then be carried out as hereinbefore set out for performing the corresponding operations in applying other insulating materials such as Formvar.

Of the insulating materials discussed hereinbefore the epoxy material is the best suited for the practice of the present invention. The polyvinyl acetyl resin has the disadvantage that the solids content (percent by weight of insulating material) of the bath used in applying these resins is limited to a low value because the viscosity of the solution reaches the highest value which it is practical to work with at a low solids content. Thus, the solids content of the polyvinyl acetal resin bath is usually not more than about 18%. Nylon has the disadvantage that nylon coating the conductor is soluble in the nylon solution which is introduced into the winding to impregnate the interstices thereof, or, if molten nylon rather than a solution of nylon in solvent is used to impregnate the coil, the molten nylon is likely to melt the coating on the conductor. The oleo-resinous insulating material has the disadvantage that it depends in part on oxidation for curing, so that in order to obtain a thorough cure, the solution containing the oleo-resinous material must be applied in a thin film and following each application the film just previously applied must be thoroughly cured. Thus, in general, if an oleoresinous material is used, in order to thoroughly impregnate the winding, the operation of forming the conductor into turns must be interrupted frequently, and the partially formed winding must be dipped and cured several times following each interruption. As solutions of epoxy resins and curing agents therefor may have a solids content as high as about 60%, and as these resins cure by polymerization to an insoluble, thermosetting plastic, they do not suffer from any of these disadvantages.

I claim:

1. An electrical device having a winding consisting essentially of a conductor wound into a coil and insulating components insulating the turns of said coil from each other and from surrounding structures, said insulating components comprising a coating film on the conductor, an impregnant filling the interstices of the winding. and an overall layer of protective insulating material, each of said insulating components being physically distinct and free from any fusion bond between each other but being substantially of the same chemical composition. whereby electrical stress gradients at the interfaces between the insulating components associated with said winding are minified.

2. A dynamo-electric machine comprising a member having a slot therein, a winding positioned with a portion thereof contained in said slot, said winding being formed from a conductor coated with an insulating material and being impregnated with an impregnant of insulating characteristics, and the portion of said winding contained in said slot being enclosed in a layer of sheet insulating material, said winding being characterized in that the coating on the conductor, the impregnant with which said winding is impregnated, and said layer of insulating material are physically distinct and free from any fusion bond between each other but are of substantially the same chemical composition so that electrical stress gradients at the interfaces between the insulation components associated with said windings are minified.

3. An electrical coil the turns thereof being formed by a conductor coated with a film of insulating material, said turns together being enclosed in a layer of sheet insulating material, the interstices of said coil being substantially completely filled with an insulating impregnant, characterized in that the coating on the conductor, the impregnant, and the enclosing layer of sheet insulating material all are physically distinct and free from any fusion bond between each other but are all of substantially the same chemical composition so that all of the insulating material associated with said coil and separating the coil from its surroundings has substantially the same dielectric constant and electrical stress gradients at the interfaces of the components of the insulation within said coil are minified.

4. A dynamo-electric machine according to claim 2 wherein all of the insulating material associated with said winding separating the turns thereof from their surroundings is the product of reaction between an epoxy resin and at least one curing agent therefor.

5. A coil according to claim 3 wherein all of the insulation material associated with said winding separating the turns thereof from their surroundings is the product of reaction between an epoxy resin and at least one curing agent therefor.

6. In the fabrication of an electrical device, in which a conductor coated with a film of insulating material is wound into a coil, the improvement which comprises insulating at least some of the turns of said coil with a sheet of protective insulating material having substantially the same chemical composition as the film of insulating material on the wire, and impregnating the coil with an insulating varnish in which the solids have substantially the same chemical composition as both the film of insulating material on the wire and said sheet of protective insulating material, and maintaining the temperature of said coil at all times below the fusion temperature of any of said insulating components, whereby said insulating components are maintained physically distinct and free from any fusion bond between each other and electrical stress gradients at the interfaces between them are minified.

7. In the construction of a dynamo-electric machine, in which a conductor coated with a film of insulating material is wound into a coil and then inserted into slots formed in a supporting structure, the improvement in insulating the machine which comprises lining the slots with a layer of sheet insulating material having substantially the same chemical composition as the film of insulating material on the wire, and impregnating the coil with an insulating varnish in which the solids have substantially the same chemical composition as both said film and said sheet material, and maintaining the temperature of said coil at all times below the fusion temperature of any of said insulating components, whereby said components are maintained physically distinct and free from any fusion bond between each other and electrical stress gradients at interfaces between the insulating components are minified.

References Cited in the file of this patent UNITED STATES PATENTS 2,392,388 Joyce Jan. 8, 1946 2,425,294 Morgan Aug. 12, 1947 2,479,400 Pecoroni et al Aug. 16, 1949 2,524,536 Nordlander et al Oct. 3, 1950 2,549,309 Hill et al Apr. 17, 1951 2,648,018 Meier Aug. 4, 1953 OTHER REFERENCES Article Typical Applications of Epoxy Resins, on pages 8487 of Rubber and Plastics Age, for February 1954.

Ethoxyline Resins, on pages 6-10 of Plastics, for January 1952. 

